Palmu played in his native Finland, as a youth with Jokipojat and Jokerit at the Junior A and B level before opting to continue his development in North America after he was selected 22nd overall in the 2014 CHL import draft by the Owen Sound Attack of the Ontario Hockey League (OHL).
Following three seasons of major junior hockey with Owen Sound and after his selection to the Vancouver Canucks in the 2017 NHL Entry Draft, Palmu opted to return to Finland to embark on his professional career, agreeing to a contract with TPS of the Liiga on 3 May 2017.
[3] After attending the Canucks 2018 training camp, Palmu was reassigned to begin the 2018–19 season with AHL affiliate, the Utica Comets.
[4] Petrus played just 12 of 28 games with Utica, featuring heavily as a healthy scratch, before he was loaned back to TPS to continue his development for the remainder of the season on 13 December 2018.
[5] Palmu again attended the Vancouver Canucks training camp prior to the 2019–20 season, before he was returned to Finland for a second consecutive year on loan, joining JYP Jyväskylä on 3 October 2019.